Mid-infrared spectra of optically-selected type 2 quasars

Abstract

Type 2 quasars are luminous active galactic nuclei (AGNs) whose central engines are seen through large amounts of gas and dust. We present Spitzer spectra of 12 type 2 quasars selected on the basis of their optical emission line properties. Within this sample, we find a surprising diversity of spectra, from those that are featureless to those showing strong polycyclic aromatic hydrocarbon (PAH) emission, deep silicate absorption at 10 μm, hydrocarbon absorption, high-ionization emission lines, and H2 rotational emission lines. About half of the objects in the sample are likely Compton-thick, including the two with the deepest Si absorption. The median star-formation luminosity of the objects in our sample measured from the strength of the PAH features is 5 × 1011 L, much higher than for field galaxies or for any other AGN sample, but similar to other samples of type 2 quasars. This suggests an evolutionary link between obscured quasars and peak star-formation activity in the host galaxy. Despite the high level of star formation, the bolometric output is dominated by the quasar in all cases. For a given strength of 10 μm Si absorption, ultra-luminous infrared galaxies are significantly colder than type 2 quasars (their F ν[14.5 μm]/F ν[27.5 μm] ratio is 0.5 dex lower), perhaps reflecting different obscuration geometries in these sources. We find that the appearance of the 10 μm feature (i.e., whether it shows in emission or in absorption) is well correlated with the optical classification in type 1 and type 2 quasars, contrary to some models of clumpy obscuration. Furthermore, this correlation is significantly stronger in quasars (L bol ≳ 1045 erg s-1) than it is in Seyfert galaxies (L bol ≪ 10 45 erg s-1). © 2008. The American Astronomical Society. All rights reserved.